Currently the secondary effluent of a sewage treatment plant often contains a certain amount of nitrate nitrogen, phosphorus and suspended solids, which can cause excessive accumulation of nutrients in a water body when directly discharged into rivers and lakes, and even more cause water bloom and the like. The national environmental protection department require urban sewage treatment plants to strictly obey a first-grade A standard as specified in the “pollutant discharge standards for urban sewage treatment plants” (GB18918-2002), which applies a higher requirement on the treated effluent of sewage treatment plants. Therefore, it is of urgent need to technically reform and upgrade the existing sewage treatment plant, so as to remove nitrate and suspended solids from the secondary effluent.
The denitrification deep-bed filter is sewage advanced treatment unit which integrates denitrification biological nitrogen removal function and filtering function as a whole, having a simple filter body structure, a low running cost, and convenient operation management. The working principle of the apparatus is that, when the sewage flows through a filtering material contained within the filter under the action of gravity, a biomembrane adhered on the filtering material reduces nitrite and nitrate into nitrogen gas under an anaerobic condition; and meanwhile under the physical interception and absorption actions of the filtering material, the suspended solids contained in the inlet water are also removed. The sewage entering the denitrification deep-bed filter is generally sewage treated through secondary biochemical treatment, which has a low COD, but most denitrifying bacteria are heterotrophic bacteria, such that for ensuring high-effective running of the filter, it needs to add a certain amount of carbon source to achieve the purpose of high-effective nitrogen removal.
Since generally dissolved oxygen (DO) is presented in secondary effluent, and the heterotrophic bacteria on the surface layer of the filtering material consumes the DO by using the carbon sources contained in the inlet water, the heterotrophic bacteria on the surface layer of the filter have a mass reproduction, which decreases the gap of the upper-layer filtering material; meanwhile due to the suspended solids as intercepted on the surface layer of the filtering material, under coaction of the gap decrease and the suspended solids, a “mud blanket” phenomenon occurs in the surface layer of the filtering material in a short period, and thus the resistance of the filter bed is increased dramatically and the treatment capacity is decreased rapidly.
To ensure treatment efficiency and reduce the cost, most of the existing method starts with strict control of the dosage of the carbon source and the separation aspect in the functions of the filter. For example, Chinese patent No. 201320039384.9, which is published at Jul. 17, 2013, discloses a patent application document named “combined-type deep-bed denitrification nitrogen-removal filter”, the invention disclosed in the document relates to a combined-type deep-bed denitrification nitrogen-removal filter, which is provided with a filter, a clean water tank, a carbon-source dosing system and a sewage tank, and the filter is also provided with a denitrification bio-bed, a quartz sand filter bed, a supporting layer, and a gas-water mixing region formed by W filter bricks. The denitrification bio-bed adopts a combination of a natural beehive-shape volcanic-rock filling material with high porosity and a quartz sand filtering material, to complete the nitrogen removal and removal of suspended matters in multiple steps. Chinese patent No. 201410140512.8, which is published at Apr. 9, 2014, discloses a patent application document named “Intelligent and Precise Carbon-Source Dosing System for Denitrification Deep-Bed Filter”, the invention disclosed in the document relates to an intelligent and precise carbon-source dosing system for a denitrification deep-bed filter, which is provided with a water inlet, a COD inlet water detector, a dosing meter pump, an inlet water meter pump, a central control system, a dissolved oxygen meter, a pH meter, a thermometer, a denitrification deep-bed filter, a COD outlet water detector, a water outlet, and a nitrate analyzer of the water outlet and inlet, wherein the carbon-source dosage is controlled through the central control system according to the collected COD, flow rate, DO, temperature and pH value signal. The aforementioned patents relieve the problem of clogging the surface layer of the filter via the separating function of the filter and precise dosing of the carbon source, but do not reduce the surface-layer clogging of the filtering material due to mass reproduction of heterotrophic bacteria as caused by the fact that heterotrophic bacteria consume the DO by using the carbon source, or due to the interception of suspended matters.